1. Field of the Invention
This invention relates to a gene expression system for producing galactosyltransferase from a recombinantly modified myeloma cell, and to method for producing galactosyltransferase using the recombinantly modified myeloma cell.
2. Description of the Related Art
The production of biologic products from cells grown in mass cultures has a long history. Before the advent of recombinant genetic technology, the selection of cells or cell lines for production of biological products was a slow and haphazard process.
Today, recombinant genetic technology allows cells to be modified by inserting genes that encode `foreign` proteins which are not normally produced by a host cell, creating cell lines that produce foreign proteins--antibodies, enzymes, hormones (Current Protocols in Molecular Biology, eds. Ausubel, F. M., et al. Chapter 16, 1994). However, it is not predictable whether any particular gene or host cell combination will lead to a useful level of production.
A typical technique for improving the yield of protein harvestable from recombinant mammalian cells involves stressing the cells by nutritional deprivation, temperature extremes, or other physico-chemical techniques. When stressed, the cell line's production of the desired material dramatically increases just prior to the death of the cells. A significant disadvantage of cell stress techniques is an increase in contaminants, especially proteolytic enzymes, that are released from the cells at the time of their death. The presence of these proteolytic enzymes may often damage the cloned protein as well as complicate procedures required to purify the cloned protein. While some proteins, such as monoclonal antibodies, are not subject to degradation by proteolytic enzymes, other biological material, in particular enzymes, are far more sensitive to the presence of proteolytic enzymes. Other disadvantages of the cell death approach to culture are that it requires extra labor to restart the manufacturing cell line after each batch death, requires cleansing from the culture system of proteolytic enzymes and other contaminants released from dead cells in the previous batch, and requires reinoculation of a fresh medium with concomitant waiting until the culture reaches sufficient cell density to initiate a stress-induced death. Consequently, production of cloned enzymes by the cell stress approach is not generally practical.
In view of the foregoing, there is a need for a gene expression system for producing enzymes and a method of producing enzymes which would allow continuous, longer-term scale-up production of enzymes which avoids the disadvantages of contamination from cultured host cells which are stressed to induce death and the down-time associated with the restarting of the fermentor culture.